5G (NR) Terminal (UE) RRC_INACTIVE State (1)

I. The RRC_INACTIVE state is a fundamental architectural innovation in 5G (NR), designed to address the critical latency and signaling overhead issues that plagued LTE networks. In 4G (LTE), frequent transitions between the RRC_IDLE and RRC_CONNECTED states of the terminal (UE) caused a huge network signaling load and introduced latency penalties during service recovery, which is particularly problematic for modern smartphone usage patterns characterized by frequent small data transmissions. The RRC_INACTIVE state bridges the gap between fully connected and fully disconnected states, enabling fast service recovery while maintaining power efficiency and reducing core network signaling.

II. The need for RRC_INACTIVE stems from the limitations of 4G (LTE) and the requirements of 5G: In 4G (LTE) networks, prolonged user inactivity triggers a transition to the RRC_IDLE state to save power. However, restoring to the RRC_CONNECTED state requires re-establishing the RRC connection, which involves a large amount of RRC signaling interaction and introduces significant latency. In modern mobile applications, terminals frequently generate bursts of small data packets (such as social media updates, instant messages, and IoT sensor data), leading to repeated "IDLE-CONNECTED-IDLE" state transitions, burdening both the radio interface and the core network.

III. The advantages of RRC_INACTIVE are threefold:

• Reduced signaling overhead: Both the UE and the gNB store the UE's access stratum (AS) context, so a complete RRC re-establishment process is not required during service recovery.
• Reduced transition latency: The state transition from INACTIVE to CONNECTED is much faster than from IDLE to CONNECTED because the radio bearer configuration is retained.
• Maintained core network connectivity: The UE remains in the CM-CONNECTED state relative to the 5G core network (5GC), meaning that the UE's connection on the NG interface between the gNB and the AMF remains active.

IV. RRC State Architecture: A 5G (NR) terminal (UE) can be in three different RRC states:

• RRC_IDLE: The RRC connection does not exist; the UE performs cell selection/reselection and listens for paging. Both the UE and the network's AS context have been released.
• RRC_INACTIVE: The RRC connection is suspended, and the AS context is retained; the UE monitors paging within the configured RAN Notification Area (RNA), and its behavior is similar to the IDLE state to save power.
• RRC_CONNECTED: The RRC connection is active and dedicated resources have been allocated; the UE exchanges user plane and control plane data.

V. Terminal (UE) Connection Management: In the 5G system, terminal (UE) connection management in the NAS (Non-Access Stratum) interacts with RRC in two states; these are:

• CM-IDLE: Corresponds to the RRC_IDLE state; there is no NG connection between the gNB and AMF;
• CM-CONNECTED: Corresponds to the RRC_CONNECTED and RRC_INACTIVE states; the NG signaling connection between the gNB and AMF remains active.